Dashboards created with node-red-dashboards all have the same favicon
You can easily change this by changing the icons in the .node-red/node-modules/node-red-dashboard/dist, but this change will be lost the next time you update node-red-dashboard.
To get a permanent change, add your icon as files icon64x64.png, icon120x120.png, and icon192x192.png to a folder of your choice, e.g. /home/nodereduser/.node-red/icons, where nodereduser is the user executing node-red.